1. Field Of The Invention
This invention relates to the detection of gases and, more particularly, to gas sampling devices which draw a sample gas to a sensor within the device.
2. Background Art
Devices for sampling a gas and determining the concentration of a particular gas in the sample gas are well-known. Such gas sampling devices generally include a gas sensor, which develops an electrical signal in response to the presence of a particular gas, and circuitry for converting that signal into an output reading. Gas sampling devices often include a pump which draws the sample gas through a probe and causes the gas to pass over the sensor.
Hand-held, battery operated gas sampling devices are also well-known. These devices are used to test for the presence of a particular gas in remote areas or other locations where it is impractical to use larger, bench devices. Hand-held gas sampling devices are commonly used by inspectors to check the operation of furnaces, gas hot water heaters, and devices in which excess concentrations of carbon monoxide, sulfur dioxide, oxygen or the like is an indication of a problem.
It is known to use a reciprocating pump to draw gas into a sampling device. However, reciprocating pumps have a number of drawbacks. Specifically, the gas flowing from a reciprocating pump exhibits pressure pulsations resulting from the reciprocation of a diaphragm internal to the pump. Such pressure pulsations interfere with the operation of many gas sensors, such as electrochemical gas sensors which are extremely pressure sensitive. Moreover, a reciprocating pump requires an expensive and powerful motor and is sensitive to dirt and liquid.
A squirrel cage type of centrifugal fan has also been used to draw a sample gas into a gas sampling device. However, such a fan develops only a low pressure potential and cannot draw a sample gas against a large pressure. In addition, the structure permits residual gas to remain after a test, which could adversely affect the reading obtained from a subsequent test. Moreover, the squirrel cage fan draws far too much gas volume and uses too much energy from the power source for the device.
It is an object of the present invention to provide a gas sampling device which positively draws gas to the gas sensor, but which overcomes the disadvantages of the prior art. It is an objective to provide a gas sampling device which requires an inexpensive motor, which maintains little residual gas, which draws gas at a steady pressure, which is insensitive to dirt and water, which is relatively maintenance free and which produces a larger pressure differential than the squirrel cage fan, yet draws only a small amount of sample gas and is energy efficient in operation.